In the field of color image forming apparatuses, a technique for registration between the respective colors is important. Misregistration may occur due to misregistrations and distortions of f-theta lenses or reflective mirrors in the case of LD (Laser Diode) raster systems, and distortions and installation errors of LEDA heads in the case of LEDA (Light Emitting Diode Array) writing. With respect to misregistrations with a bending and a skew in a sub-scanning direction, among misregistrations, there are a mechanical correcting way and a correcting way based on the image processing. According to the mechanical correcting way, the correction is implemented by providing an adjustment mechanism for displacing the mirror in the writing unit. An actuator such as a motor is utilized to automate the adjustment.
According to the correcting way based on the image processing, parts of the image data are accumulated in a line memory, and the lines of the line memory from which the data is to be read are switched according to the writing positions, thereby shifting the image in the sub-scanning direction and thus correcting the skew between colors. In this case, it is known that the skew may be preferably reduced by adding a line memory in the image processing part according to a range to be corrected.
The way of reducing the skew is known from Patent Document 1, for example. Patent Document 1 discloses a method of correcting an offset and a skew in a printer device which includes a detecting part configured to detect a status of a paper sheet supplied between a waiting roller for supplying the paper sheet and a photosensitive drum; and a controlling part configured to receive the detection signal from the detecting part, perform the correction and output a control signal of an optical system and a driving signal of a motor for adjustment of the optical system. According to the method disclosed in Patent Document 1, a print start position and print start timing are corrected based on an offset which is detected based on the detection signal from the detecting part, a skew angle is calculated based on the detection signal, and the position of the optical system is adjusted such that the skew angle is corrected.    [Patent Document 1] Japanese Laid-open Patent Publication No. 04-189239
However, with the correcting method according to the related art, even if the skew amounts between the colors are corrected, there is a problem that the start positions of writing of the image are not parallel with a horizontal (main) scanning direction if the paper sheet is skewed. Further, with a line memory control for the skew correction, there is a problem that the lack of the capacity of the line memory may occur, and thus the correction method not be performed correctly when the correction of the skew amounts between colors and the correction of skew between the paper sheet and the image are performed in combination.
The configuration disclosed in Patent Document 1 cannot solve these problems.
An object of the present invention is to reduce the skew amount between the paper sheet and the image even with the line memory.